We investigated the litter-fall) dynamics in rain forests in a matrix of two altitudes (approximately 800 and 2000 m) and two geological substrates (sedimentary versus ultrabasic rocks) on Mount Kinabalu, Borneo, during the 1996 masting year. The two forests at 800 m were hill rain forests, and those at 2000 m were lower montane rain forests. Litter was collected at 2-week intervals with 20 litter traps in each forest for 10 months. Significant variations were observed among the dates of collection for the leaf litter fraction in all forests (P<0.00001, ANOVA), indicating that leaf litter-fall was highly seasonal irrespective of altitude and substrate. The variation of the flower/fruit litter was significantly seasonal in the two hill forests (P<0.005), but was not significant in the two montane forests (P>0.05). A distinctly dry spell with depressed (3-4 °C colder) minimum air temperatures appeared in March throughout the slope, which preceded the peak of leaf litter- fall. However, the peak leaf litter appeared one month later in the wettest montane forest on sedimentary rock than in the other forests, suggesting that the response to a trigger factor might be a function of moisture. We suggested that aridity might cause the seasonality of leaf litter-fall. However, we could not find a single proximate cause for flower/fruit dynamics during the masting year.